1. Field of Invention
Silicon is, at present, the most important material in modern semiconductor technology and is finding increased use in solar cells for the photovoltaic generation of electricity. In view of the importance of the solar cell application, the stringent requirements for purity and low cost and further in view of the orientation of the work done, the process and apparatus is described primarily in the context of production of silicon and silicon articles suitable for solar cell fabrication. However, it is to be understood that both the process and apparatus used are generally useful in the production of silicon in any desired shape for whatever end use, as well as other transition metals such as Ti, Zr, Hf, V, Nb and Ta.
A major deterrent to the development of practical solar photovoltaic systems is the cost of high purity silicon. With todays technology, approximately twenty percent of the total cost of a silicon solar cell is ascribed to the silicon material alone. That is, the cost of the silicon material produced by the conventional hydrogen reduction of chlorosilanes constitutes at least twenty percent of the cost of producing the cell. It is estimated that the cost of the silicon must be reduced by almost an order of magnitude before silicon solar photovoltaic panels will prove to be economically feasible as a power source. The fact that the chlorosilane processes require multiple separations, are so energy intensive and require such large capital investments indicate that cost of the silicon can not be reduced sufficiently to make silicon solar cells economically feasible without a major process change. Moreover current solar cell technology is based on the use of Si wafers obtained by slicing large Czochralski crystals or float-zone ingots, using singleblade inner diameter diamond saws. Nearly 30 to 40% of the silicon is lost as sawdust in the slicing process. In order to decrease the material lost in sawing and increase the number of silicon wafers produced from a single crystal or ingot, techniques would have to be developed using multiple saws with thin sawblades, sawing wires, or other advanced processes to produce thinner wafers. The difficulties or problems in cutting thinner crystal wafers increase with crystal diameter.
Therefore, considerable material, energy, and capital equipment could be saved if a process could be developed for forming silicon wafers in a mold. As a consequence, an approach to the production of solar grade silicon and silicon articles (wafers) suitable for fabrication into solar cells which is less complex, less energy intensive and which requires less capital equipment would be most desirable.